Cavity resonator for microwave electron beam tubes



June 29, 1954 L. s. NERGAARD CAVITY RESONATOR FOR MICROWAVE ELECTRON BEAM TUBES 2 Sheets-Sheet 1 Filed Dec. 1, 1948 and Y I y B III ATTORNEY June 1954 1.. s. NERGAARD 2,682,622

CAVITY RESONATOR FOR MICROWAVE ELECTRON BEAM TUBES Filed Dec. 1, 1948 2 Sheets-Sheet 2 INVENTOR ATTORNEY lggllgwd I Patented June 29, 1954 CAVITY RESONATOR FOR MICROWAVE ELECTRON BEAM TUBES Leon S. Nergaard, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 1, 1948, Serial No. 62,804

21 Claims.

This invention relates to cavity resonators, and more particularly to cavity resonators for use in electron beam modulating tubes.

Electromagnetic energy absorption tubes are known in which a beam of electrons is directed through an electromagnetic cavity resonator. The beam derives energy from the electric field of the resonator. This derived energy is dissipated upon impingement of the beam on an anode or collecting electrode. The energy is supplied to the resonator by appropriate exciting means. This energy may be taken from a suitable source. Such tubes are useful, for example, as energy absorption modulators. As an example of a typical utilization of such an energy absorption tube, reference may be made to the copending application of John S. Donal, Jr., and Robert R. Bush, Serial No. 757,755, filed June 28, 19%7, and entitled Modulated Microwave Generators, now Patent No. 2,602,156, dated July 1, 1952. The resonator shown in the above-identified application includes a coaxial cavity resonator having an outer tubular wall, and two opposed semicircular inner members or deflection plates coaxial therewith. The deflection plates are spaced radially and axially from the outer wall and supported by metallic supports or stubs extending radially therefrom. The outer wall is capped by end plates which are apertured for passage of the beam. On excitation of the cavity resonator, as by a loop fed from a coaxial line, an alternating electric field is created between the deflection plates and transverse to the direction of beam travel. A substantially uniform magnetic field is impressed parallel to beam and to the longitudinal axis of the resonator. The magnetic field strength H is so adjusted that w=He/m where w is the angular operating frequency and e/m is the charge-to-mass ratio of the electrons in the beam. With the magnetic field so adjusted and with a truly transverse electric field, it is known that the electrons will individually travel in spiral paths with the same angular velocity equal to the angular velocity w of the electric field and absorb energy continuously from the electric field, thereby increasing their total velocity and kinetic energy. The Velocity component in the initial direction of travel along the axis of the resonator remains unchanged. As a result, the beam itself consists of a pencil line of electrons which revolves about the axis and describes a cone. When the axial transit time of all the electrons is constant, the energy absorbed is proportional to the field strength squared, i. e. the device acts as pure resistance and may be matched to a line.

'2 With the resonator described in the abovementioned copending application and with other prior resonators used in such type tubes, the electron beam is modulated longitudinally by longitudinal electric fields between the ends of the cavity resonator and the ends of the deflection plates. This longitudinal modulation varies the longitudinal transit time. Then the power absorption is no longer proportional to the square of the applied field, i. e., the device does not behave as a resistance and will not properly terminate a line. It is sometimes desired in energy absorption type tubes utilizing cavity resonators to employ a cavity resonator having a certain axial length fixed by the operating frequency, whereas because of the tube dimensions and requirements for compactness, the resonator is required to be of a shorter axial length. It is difficult to satisfy these conflicting requirements with regard to the length of the resonator. Another difliculty with prior art resonators for use in absorption tubes has been that the electric field is not uniformly transverse to the path of electron travel, that is, that the electric vectors throughout the path of travel are not parallel to each other. Such parallelism is a desired and desirable feature in order to obtain the desired spiralling efiect of the electrons and improvements in such uniformity of the electric field vector directions are of importance. Further, spurious field distributions are sometimes excited in these cavity resonators which are undesirable and adversely affect the efiiciency of operation of the tube.

It is an object of the present invention to provide a cavity resonator having a highly desirable field distribution characteristic and mode of excitation for use in energy absorption tubes.

It is a further object of the invention to improve such cavity resonators.

It is another object of my invention to provide a cavity which in combination with an absorption tube will act as a pure resistance which may be used to terminate a line.

Another object of the invention is to prevent the excitation of spurious undesired modes in such cavities. I

It is another object of the invention to provide such cavity resonators which may resonate at a frequency lower than that indicated by the physical length of the resonator.

It is a further object of the invention to provide a cavity resonator for an energy absorption tube the electric field of which will be substantially uniform throughout the electron traveled portion of the resonator.

These and other objects, advantages, and novel features of the invention will be more apparent from the following description and the accompanying drawing in which like numerals refer to like parts and in which:

Figure 1 is a perspective partially schematic view of an energy absorption type tube showing a cavity resonator of the invention in place;

Figure 2 is a perspective view of the resonator of Figure 1 by itself;

Figure 3 is a transverse cross-sectional view of the resonator of Figure 2 taken along the lines 3-3 of Figure 2;

Figure 4. is a perspective View of another more compact resonator of the invention which may be used in place of the resonator of Figures 2 and 3;

Figure is a transverse cross-sectional view of the resonator of Figure 4 taken along the lines 5& of Figure 4;

Figure 6 is a cross-sectional view of still another resonator of the invention which is tunable and which is physically shorter than its electrical length; and

Figure 7 is a longitudinal cross-sectional view of the resonator of Figure 6 taken along the lines ll of Figure 6.

The foregoing objects are achieved, in accordance with the invention, by providing a cavity resonator having walls defining inner and outer resonator spaces, the inner space at least being substantially symmetrical with respect to a longitudinal axis along which the electrons or" the electron beam are to be directed on entering the inner resonator space and around axis which they spiral in their travel therethrough. The outer and inner spaces communicate through longitudinal slots in the inner space walls, the slots being on opposite sides of the axis. The outer space walls may terminate on or contact the inner space walls so that the high frequency lines of magnetic force are prevented from encircling the inner chamber. Thus all of these lines are constrained to enter the inner space from the outer space through the slots in the inner space walls. I have found that the efficiency of coupling the resonator fields with the electron beam is thereby increased, and spurious modes having lines of magnetic force encircling the inner chamber are thereby suppressed. The resonator has end plates to short circuit the inner and outer chamber walls and which end plates are closures for the outer resonator space. With the structure as described, I have further found that longitudinal electric fields are substantially completely suppressed. Thus the undesired longitudinal modulation of the electron beam, which might cause bunching and undesired modulation of the output together with undesired reactance effects on. the line from which energy is to be absorbed, is avoided.

Referring now more particularly to Figure 1 which is a perspective view of an absorption tube having one form of cavity resonator embodying the invention, an evacuated glass envelope H encloses an electron beam gun comprising a thermionic cathode 13, a control grid IS, a screen grid H, and a focusing or accelerating electrode l9. The electron gun elements have suitable voltages applied thereto to project the electron beam 2| to a collector electrode 23 having a voltage applied thereto to capture the electrons of the beam. Envelope H is surrounded by a cavity resonator 25 devised according to the invention.

Referring now also to Figures 2 and 3, which are, respectively, a perspective view and a transverse crosssectional view of cavity resonator 25, an inner resonator space 2i is defined by two elongated, substantially semicylindrical members or deflection electrodes 29 and 3|. Members 29 and 3| may be considered together as a tubular member having longitudinal slots 33 and 35. The slots 33 and 35 extend substantially the full length of the resonator, which in this instance is substantially a half wavelength electrically and physically at the operating frequency. Members 29 and 3% exhibit symmetry with respect to a longitudinal axis. Thus the inner resonator space may be considered as substantially symmetrical with respect to said axis, open at the ends for entry and exit of an electron beam, the walls of the inner resonator space having longitudinal slots to afford communication with the outer resonator space. The outer resonator space 37 is defined by the inside of an outer circularly symmetrical wall 39 and the outside of the tubular member 29-31 and also by a partition wall 4! which contacts inner member 29 midway between the slots and extends radially to contact the outer member 39 which is substantially coaxial with the structure. Additionally, the outer resonator space 371 is bounded by end plates 43 and 15. The end plates are in the shape of apertured discs and close the outer resonator space by contacting the walls of the inner space and the walls of the outer space and the partition. The end plates are apertured coextensively with the end openings of the inner space. As will be well understood by those skilled in the art, it is the inner conductive metallic surfaces exposed to the resonator spaces by the walls and end plates which define the resonator spaces. Partition 4! extends between the members 29 and 39 along the full length of the resonator structure in a radial plane passing through the axis. The opposite faces of partition ll form two walls of the outer resonator space 3?.

When the resonator 25 is tuned to the operating microwave frequency, there is provided a transverse microwave electric field between semicylindrical members or deflection electrodes 293| in the inner resonator space 2?. This field is illustrated by the vectors 4? of Figure 3. With a magnetic field H adjusted between the poles NS of Figure l to provide a substantially uniform magnetic field in the resonator space such that w=He/m, the electron beam 21, as will appear from Figure 1, will follow the directrix of a hollow cone having a radius determined by the magnitude of the energy absorption. Energy may be delivered to the resonator from a coaxial transmission line file as shown in Figure 2.

In the absence of the partition GI, resonator 25 would tend to oscillate in the dominant mode of a half-wavelength coaxial resonator. Much of the energy of excitation would be then dissipated in this dominant mode which has radial electric vectors and circumferential high frequency lines of magnetic force. The energy so dissipated would not be subject to absorption by the electron beam, as desired to produce modulation. Partition ill serves as a barrier to the encirclement of the inner resonator space 21 by these high frequency lines or" magnetic force. All such lines, as illustrated by the dotted lines 5! of Figure 3, are therefore constrained to pass through communicating slots 33 and 35 from the outer resonator space into the inner resonator space. Thus the, resonator is constrained to operate in a mode most advantageous for the desired result of deriving an electric field having substantially symmetrical electric vectors along the axis of the inner space and transverse to said axis. In the first place, the electric field lines 41 must cross the lines I orthogonally. This assures their conformity in the inner resonator space 21 substantially as illustrated in Figure 3. Also, because of the large number of lines of magnetic force entering and passing through the inner resonator space 21, the resultant field is intense compared to the fields produced in prior art resonators. It is the intensity or strength of these fields in the inner resonator space which determine the interaction with the electron beam and its consequent energy absorption. Moreover, the resonator 25 has substantially no longitudinal electric field within the inner resonator space. Fringing within that space is substantially prevented by the short circuiting end plates or discs.

Referring now more particularly to Figures 4 and '5, which are respectively a perspective view and a transverse cross-sectional view of another cavity resonator 6| of the invention which may be substituted for the resonator 25, an inner resonator space 63 is defined by a circular tubular member comprising two substantially semicylindrical members 65 and 61. The tubular metallic members 65 and 61 may be considered as a hollow cylinder with longitudinal slots 69 and II extending substantially the entire length of the resonator. As before, the inner resonator space 63 is substantially symmetrical with respect to a longitudinal axis. An outer resonator space I3 communicates with inner resonator space 63 through slots 69 and II and has for its tubular walls a substantially semi-cylindrical metallic member I5 and two substantially coplanar walls I1 and I9 contacting the edges of the member 95 at the slots 99 and II and extending outwardly substantially in a radial plane. Walls TI and I9 may be considered as a single partition having an angular thickness to expose to the outer resonator space two metallic surfaces substantially coplanar and extending from the same sides or edges of the slots, that is, from the edges both on the one side of the axial plane through the slots. The surfaces thus are separated by substantially 180 degrees about said axis. In a cross-section taken in a plane normal to the axis, as in Figure 5, the outer resonator space is bounded substantially by two concentric semi-circles connected at the ends by radial lines. Again, the walls I! and I9 form a barrier to the encirclement of inner cavity resonator space 63 by the high frequency lines of magnetic force -8I as illustrated and as will be fully apparent from Figure 5. The solid lines in Figure 5 represent in a qualitative way the configuration of the electric field vectors within the inner cavity resonator space 63. Again, end plates 83 and 85 form a closure for the outer resonator space and contact the outer and inner resonator space walls leaving open, however, the ends of inner resonator space 63 for the passage therethrough of the electron beam. Energy is supplied the resonator from a coaxial line 86 the inner conductor of which terminates in a probe 89a. inserted appropriately in the outer resonator space I3 to excite the desired mode.

The cavity resonator of Figures 4 and 5 is preferred over that of cavity resonator 25 of Figures 1, 2 and 3, because it is more economical of space and material, more readily constructed and assembled, and is substantially similar in results.

Referring now more particularly to Figures 6 and 7 which are, respectively, a transverse crosssectional view of still another resonator of the invention, and a longitudinal cross-sectional view thereof, an inner resonator space 93 is defined Y as before by a circular tubular member comprising two substantially semi-circular members 65 and 61. An outer resonator space is defined by tubular walls including a substantially semicylindrical wall I5 and planar walls 11 and I9 the latter contacting the edges of the member 65 at the longitudinal slots 69 and II. As in Figs. 4 and 5, the walls I1 and I9 extend outwardly substantially in a radial plane and'their surfaces are separated by substantially degrees about the axis of the inner tubular member. The embodiment of Figures 6 and 7 differs from that of Figures 4 and 5 by having wings or vanes 91 and 99 on the edges of member 61 and extending substantially parallel to and in capacitive relationship with walls 11 and I9. Vanes 91 and 99 do not extend longitudinally the entire length of the resonator structure; Their object is to add capacity between the members 65 and 61 thereby to decrease the resonant frequency of the resonator for a given axial physical length. I have found that the addition of such vanes does not materially alter the uniformity of the field distribution within inner resonator space 93. Moreover, in Figures 6 and '7 there is provided a tuning member I9I having a head thereon I93, in this case circularly symmetrical with the axis of screw I95 of which it may be an integral part. The axis of the screw I95 and head I93 extend radially from the axis of the inner resonator space 93 and symmetrically midway between slots 69 and II in capacitive relationship with member 61. The screw may be worked in and out to give a smooth variation of the resonant frequency of the resonator because of its symmetrical structure and location. It should make good contact with outer space wall I5. The resonator may be supplied with energy from a coaxial line I9I terminating in an inductive loop I99. The end plates III and I I3 provide a closure for the outer resonator space 95 leaving the ends of the inner resonator space 93 open for the passage therethrough of the electron beam.

Again, the outer resonator walls are arranged to form an effective barrier against the encirclement of the inner resonator space by high frequency lines of magnetic force. The operation of the resonator of Figures 6 and 7 will be clear from what has been said hereinbefore with respect to the resonator 25 and the resonator BI.

The advantage of the resonator of Figures 6 and 7 is that it may be tuned, and further that its physical length is shorter than its electrical length by a desired amount dependent on the capacitive effect of the wings 91 and 99.

It will be apparent from the above description of the invention that I have devised cavity resonators having particularly desirable field configurations for an energy absorption beam tube, in which spurious modes are suppressed, and which resonators may be compact and economical in construction and efiicient in operation.

I claim as my invention:

1. A cavity resonator comprising an inner tubular metallic member afiording an inner surface substantially symmetrical with a longitudinal axis and consisting of two opposed semitubular parts separated by two longitudinal slots on opposite sides of said axis, an outer metallic member radially spaced from said inner member,

metallic partition means extending between and contacting said inner and outer members and longitudinally thereof, and metallic plates at the ends of the said members and said partition means and short-circuiting said members and said partition means, said inner and outer members, said metallic partition means and said metallic plates defining a single closed resonator space partially surrounding said slotted tubular member.

2. The cavity resonator claimed in claim 1, said partition being substantially planar and extending substantially in a radial plane, an extension of said plane including said axis.

3. A cavity resonator comprising an inner tubular metallic member affording an inner surface substantially symmetrical with a longitudinal axis and consisting of two opposed semi-tubular parts separated by two longitudinal slots extending substantially throughout the length of said member on opposite sides of said axis, an outer metallic member radially spaced from said inner member, metallic partition means extending between and contacting said members and longitudinally thereof, and planar metallic plates at the ends of the said members and said partition means short-circuiting said members and said partition means, said inner and outer members, said metallic partition means and said metallic plates defining a single closed resonator space partially surrounding said slotted tubular member.

4. The cavity resonator claimed in claim 3, wherein said resonator is axially electrically onehalf Wavelength at the operating frequency thereof.

5. The resonator claimed in claim 4 wherein said partition means comprises two metallic surfaces exposed inwardly to said resonator and on one side of said slots and in substantially the same radial plane passing through said axis.

6. The resonator claimed in claim 5, having two metallic vanes in capacitive relationship with said surfaces and contacting said inner tubular member on the other side of said slots, said vanes being shorter than the axial length of the cavity resonator.

7. The resonator claimed in claim 1, and further comprising a metallic body contacting said outer member and adjustable in capacitive relationship with said inner member.

8. A cavity resonator comprising an inner tubular metallic member affording an inner surface substantially symmetrical with a longitudinal axis and consisting of two opposed semi-tubular parts separated by two longitudinal slots extending substantially throughout the length of said member on opposite sides of said axis, an outer metallic member radially spaced from said inner member and coaxial therewith, a planar metallic partition extending between and contacting said members and longitudinally thereof, and metallic plates at the ends of the said members short-circuiting said members and said partition and circularly apertured coaxially and coextensively with said inner tubular member, said resonator being axially electrically and physically one-half wavelength at the operating frequency.

9. A cavity resonator having metallic walls defining an irmer resonator space and walls defining an outer resonator space and having a longitudinal axis, said inner space being substan- 8. tially symmetrical'with respect to saidaxis and being open at the ends for entry and exit of an electron beam, the walls of said inner space having two longitudinal slots on opposite sides thereof afiording communication for high fre quency lines of magnetic force between said spaces and forming a pair of deflecting elements therebetween, an outer space wall contacting an inner space wall along the length thereof and affording a barrier to the encirclement of said inner space walls by high frequency lines of magnetic force, and metallic closure members closing the ends of said outer resonator space.

10. A cavity resonator having metallic walls defining an inner resonator space and an outer resonator space and having a longitudinal axis, said inner space being substantially symmetrical with respect to said axis and having openings at the ends for entry and exit of an electron beam, the walls of said inner space having two longitudinal slots on opposite sides of said axis affording communication for high frequency lines of magnetic force between said spaces and forming a pair of deflecting elements therebetween extending the length of said resonator, at least one of said outer space walls contacting an inner space wall along the length thereof and affording a barrier to the encirclement of said inner space walls by high frequency lines of magnetic force, said resonator having metallic closure end plates short-circuiting said inner and outer space walls and apertured coaxially with said inner space to have openings coextensive with said openings at the ends of said inner space.

11. The resonator claimed in claim 10, said closure plates being planar.

12. A cavity resonator having metallic walls defining an inner resonator space and an outer resonator space and having a longitudinal axis, said inner space being substantially symmetrical with respect to said axis and having openings at the ends for entry and exit of an electron beam, the walls of said inner space having longitudinal slots on opposite sides of said axis aifording communication for high frequency lines of magnetic force with said outer resonator space, two of said outer space walls contacting an inner space wall along said slots and aifording a barrier to encirclement of said inner space walls by high frequency lines of magnetic force, the ends of said resonator having metallic end plates short-circuiting said inner and outer space walls, said outer resonator space in a plane cross section normal to said axis is bounded substantially by two concentric semi-circles connected at the ends by radial lines.

13. The resonator claimed in claim 10, further having metallic vanes extending radially from the walls of said inner space at said slots and capacitively facing one of said outer space walls and being shorter axially than said resonator.

14. The resonator claimed in claim 10, further comprising an adjustable capacitive tuning member extending through and contacting one of said outer resonator space walls and extending in capacitive relationship with an inner space wall.

15. An electrical cavity resonator comprising elongated inner and outer conducting walls defining inner and outer resonator spaces, said inner wall consisting of two opposed parts separated by longitudinal slots, and a pair of apertured conducting closure members electrically connected to the ends of said inner and outer walls for electrically closing the ends of said outer space.

16. An electrical cavity resonator comprising elongated inner and outer conducting walls of substantially equal length defining inner and outer resonator spaces, said inner wall consisting of two opposed parts separated by two longitudinal slots extending substantially throughout the length of said inner wall, and a pair of apertured conducting closure members mechanically and electrically connected to the ends of said inner and outer walls for positioning said walls relatively to each other and for electrically closing the ends of said outer space.

17. In combination: a cavity resonator comprising elongated inner and outer conducting walls defining inner and outer tubular resonator spaces, said inner wall having two longitudinal slots on opposite sides thereof dividing said inner space wall into two opposed deflecting electrodes, and a pair of apertured conducting closure members each electrically connected to one end of said outer wall and to the adjacent ends of said deflecting electrodes for electrically closing the ends of said outer space; and an electron beam tube extending through the apertures in said closure members and comprising means for supplying and directing a beam of electrons longitudinally through said inner resonator space and between said deflecting electrodes, and means for collecting said beam.

18. A cavity resonator according to claim 15, wherein the length of said longitudinal slots is substantially one-half wavelength at the operating frequency. v

19. A cavity resonator comprising, a first conducting wall means providing a first resonator space, a second conducting wall means spaced from and at least partially surrounding said first wall means, conducting partition'means extending between said two wall means and longitudinally thereof and forming therewith a second resonator space, said first wall means having longitudinal slots providing communication between said two spaces, and other conducting wall means connected to the ends of said first and second wall means and closing the ends of said second space, said first wall means comprises a pair of elongated, substantially semi-cylindrical conducting members spaced apart at the sides and connected together at the ends thereof to provide said slots.

20. A cavity resonator according to claim 19, where-in the length of said slots is substantially one-half wavelength at the operating frequency.

21. A cavity resonator comprising a tubular first elongated conducting member providing a first resonator space therein, a second elongated conducting member spaced from and surrounding at least half of said first member, at least one conducting partition extending between said members and longitudinally thereof and forming therewith a second resonator space, the wall of said first member consisting of two opposed semitubular parts separated by two longitudinal slots on opposite sides of said member, said slots providing communication between said spaces for lines of magnetic force, and conducting end plates extending between said members at the ends thereof to close the ends of said spaces.

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